The invention is based on an adjusting device for a throttle valve in a fuel preparation system of an internal combustion engine.
In a known adjusting device of this type, a so-called throttle valve adjuster (German Patent 28 12 292), the servomotor is embodied as a two-phase synchronous motor with a permanent magnet rotor. The two winding phases are wound on the stator in such a way that when the current direction is the same they generate magnetic fields that are phase-offset from one another by 90.degree.. The control device generates a pulse train of constant frequency, and the pulse width is varied as a function of engine operating parameters. This pulse train is fed both directly to one wind phase and also in inverted fashion to the other winding phase. The result is magnetic fields of different intensity that are rotated by 90.degree. from one another; the resultant magnetic field can assume rotational positions between 0.degree. and 90.degree.. Depending on the resultant magnetic field, the rotor and thus the throttle valve are moved into an associated rotary position.
In another known adjusting device of this type (German Patent Application 38 14 702, U.S. Pat. No. 4,951,772), the servomotor is embodied as an electronically switched direct current motor, the driven shaft of which is coupled via a gear wheel and belt drive to the throttle valve shaft carrying the throttle valve. In turn, the rotor of the DC motor is provided with permanent magnet poles. Via the toothed belt drive, the DC motor effects an adjustment of the throttle valve shaft counter to a restoring spring that engages it.
Another known adjusting device for a throttle valve (German Patent Application 30 13 984) includes a rotary magnet arrangement instead of an electric servomotor; axially parallel air gaps are formed between the poles of the disk-segment-shaped armature and the magnet poles of the magnet housing of the electromagnet. To generate constant torque over the angle of rotation of the armature, which is joined in a manner fixed against relative rotation to the throttle valve shaft, when the current acting upon the coil is constant, the end faces of the armature poles are embodied as wedge-shaped. Such an adjusting device is relatively complicated in structure, since given the fact that an actual-value transducer for the throttle valve position is dispensed with, the wedge shape of the armature poles must be embodied very accurately, if an unequivocal association between the throttle valve position and the current intensity acting upon the coil of the electromagnet is to be obtained.